1. Field of the Invention
The present invention relates to an image formation apparatus which uses a plurality of light emitting elements and forms an image on a photosensitive body and, more particularly, to an image formation apparatus in which elements such as light emitting diodes (LEDs) are arranged in one or a plurality of arrays and these elements are selectively driven to form a dot image on the photosensitive body.
2. Description of the Prior Art
FIG. 1 schematically shows the configuration of an LED printer for forming an image using a plurality of LEDs.
A photosensitive drum 101 is driven to rotate in the direction indicated by arrow a. A primary charger 102 uniformly charges the surface of the photosensitive drum 101. An LED printer head 103 forms a light spot on only a portion of the surface of the photosensitive drum 101 from which the charge is removed. The charge on the remaining portion of the surface of the photosensitive drum 101 remains. In other words, an electrostatic latent image is formed. When the image-bearing surface of the drum 101 passes by a developing unit 104, the toner is attached or not attached to the image or non-image portion of the surface of the drum 101 in accordance with the surface potential, thereby forming a visible or toner image on the surface of the drum 101. In the process described above, whether or not the toner is attached on the portion of the drum 101 irradiated with a light spot by the LED printer head 103 can be arbitrarily determined in accordance with the polarity of the charger 102 and the polarity of the toner held in the developing unit 104. The toner image passed through the developing unit 104 is transferred by a transfer charger 105 onto a paper sheet supplied from a cassette 106 or 107. When the paper sheet is passed through a fixing unit 108, the toner image on the drum 101 is fixed. A cleaner 109 removes the residual toner from the drum 101, and a discharge lamp 110 discharges the residual charge on the drum 101.
FIG. 2 is a perspective view of an LED array board 201 constituting the LED printer head 103. A substrate 202 serves as a radiating fin, and wiring means 203, 204 and 205 comprise ceramic substrates. Cables 206 and 207 transmit image signals and are connected to a power source. LED array chips 208-l to 208-n have an array of LEDs at their centers. Drivers 209-l to 209-n and 210-l to 210-n are drivers for the LED array chips 208-l to 208-n, i.e., LED drive integrated circuits (to be referred to as LED ICs hereinafter) having built-in serial-to-parallel converters of the image signals received through the cables 106 and 107, and the like. FIG. 3 shows an enlarged portion of the LED array chip 208-m and the LED drive ICs 209-m and 210-m. LEDs 301-1, 301-2, 301-3, 301-4 and so on are arranged in an array at substantially the center of the LED array chip 208-m. The odd-numbered LEDs 301-1, 301-3 and so on are wired to the upper side, while the even-numbered LEDs 301-2, 302-4, and so on are wired to the lower side. The LEDs are then wire-bonded to LED drive terminals 302-1, 302-2, . . . , 303-1, 303-2, . . . +# of the LED drive ICs C209-m and 210-m.
The LED array board 210 has the arrangement as described above. After image signals of one array are supplied to the LED drive ICs 209-l to 209-n and 210-l to 210-n through the cables 206 and 207 and the data of one array is shifted, the shifted data is parallel-produced to the LED drive terminals 302-1, 302-2, . . . , 303-1, 303-2, . . . . Then, the respective LEDs are turned on or off to generate light spots corresponding to image signals of one array.
FIG. 4 shows the relationship between the LED light emitting portion and the drum image forming point. The LED array chip 208-m forms an image on the photosensitive drum 101 through an imaging optical system 401 such as a focusing light guide. Light rays L1 from the LED 301-1 at an angle .theta. become light ray L(1-t) in the imaging optical system 401 when the angle .theta. is small. However, when the angle .theta. is increased, the light rays are partially cut off. Only the light rays L(1-t) which are incident on the system 401 are transmitted to the drum 101 and form an image. The light distribution characteristics of the LED 301-1 are as shown in FIG. 5; the flux density is high up to a region wherein the angle .theta. is relatively great. Assuming that the light rays emitted from a light emitting element having light distribution characteristics substantially equivalent to a sphere become incident on the system 401 when the angle is up to .theta. and do not become incident on the system 401 when the angle exceeds .theta., the ratio of the output optical energy of the light emitting element to the input energy thereto is calculated and is shown in Table 1 below:
TABLE 1 ______________________________________ Angle .theta. (.degree.) 10 15 20 25 30 35 40 45 ______________________________________ Efficiency (%) 5.9 12.9 22.0 32.5 43.8 55.0 65.6 75.0 ______________________________________
With an imaging optical system 401 currently available having a relatively great angle .theta., distances li and lo are 3 mm, distance lc is about 9 mm, and .theta. is about 15.degree., in FIG. 4. Therefore, even light attenuation through the imaging optical system 401 is not considered, only about 13% of the total optical energy emitted from the LED 301-1 reach the drum 101.
FIG. 6 shows the LED light emitting portion including an LED chip 501, an effective light emitting surface 502, an electrode 503, and a connecting surface 504 between the electrode and the LED. As has been described above, the LED has the light distribution characteristics as shown in FIG. 5 when the direction perpendicular to the effective light emitting surface 502 is determined as 0.degree.. The light output is assumed to be substantially proportional to the current density at the p-n junction below the effective light emitting surface 502. Since the LED light emitting surfaces corresponding to respective pixels are arranged in an array, one side of the effective light emitting surface 502 of each LED is set to be smaller than the pixel pitch so as to allow discrimination of boundaries between the adjacent pixels. For example, in an LED array having 10 pixels per mm, the pixel pitch is 100 .mu.m, while the effective light emitting surface has dimensions of 80 .mu.m.times.80 .mu.m. Thus, 20 .mu.m is set at a boundary between the pixels. Meanwhile, in order to form a clear digital image, 16 or more pixels must be formed per mm. In this case, the pixel pitch becomes 62.5 .mu.m, and the effective light emitting surface has dimensions of about 40 .mu.m.times.40 .mu.m. In this manner, when the pixel density is increased from 10 pels/mm to 16 pels/mm (1.6 times), the effective light emitting surface area is reduced to 1/4 the original area.
As shown in FIG. 2, in a system wherein drive ICs are mounted together with LED array chips and each of the LED arrays is simultaneously driven, if the process speed is kept the same, the ON time for one pitch when the pixel density is 16 pel/mm is shortened to 1/1.6 as compared to the case of the pixel density of 10 pels/mm. Therefore, if the drum sensitivity is kept the same, the illuminance per unit area when the pixel density is 16 pels/mm must be 1.6 times that with the pixel density of 10 pels/mm. When the pixel density is 16 pels/mm, the area per pixel is (1/1.6).sup.2 that when the pixel density is 10 pels/mm. Therefore, the light emission output per pixel of the light emitting portion having a pixel density of 16 pels/mm becomes 1/1.6 that of a light emitting portion having the pixel density of 10 pels/mm. Therefore, as has been described above, when the pixel density is 16 pels/mm, the effective light emitting surface area is 1/4 that when the pixel density is 10 pels/mm. Therefore, the current density of each LED in a light emitting portion having a pixel density of 16 pels/mm is 2.5 times that of an LED in a light emitting portion having a current density of 10 pels/mm since (1/1.6)/(1/4)=2.5
In order to reduce the number of drivers used and to reduce the cost, a drive method is often used wherein an n.times.m matrix of LED array chips is arranged, and an array of n LEDs are simultaneously driven by m time division. In this method, when the pixel density is increased from 10 pels/mm to 16 pels/mm and the number of LEDs to be driven simultaneously is determined to be n, the time division number becomes 1.6 times that when the pixel density is 10 pels/mm. Therefore, in this case, if the process speed is kept the same, the time corresponding to one pitch, i.e., the time for forming one dot is shortened to (1/1.6).sup.2 as compared to that when the pixel density is 10 pels/mm. Therefore, in this case, the light emission output per pixel of the light emitting portion becomes the same for either pixel density of 10 pels/mm or 16 pels/mm, and the current density for each LED when the pixel density of 16 pels/mm becomes four times that when the pixel density is 10 pels/mm.
The higher the current density, the lower the light emission efficiency of an LED. The higher the current density, the shorter the life of the LED. In addition, it is known that due to various problems encountered in the manufacture when the pixel density is increased, the light emission efficiency is reduced.
More specifically, when the pixel density is increased, the light emission efficiency is decreased. Then, the current density must be increased. As has been described above, in order to increase the pixel density to 1.6 times the original value, the current density must also be increased to 2.5 to 4 times the original value. With such an increase in the current density, the life of LEDs is reduced.
In this manner, when an image of an LED light emitting surface is formed on a drum surface by an imaging system using an LED array chip, only about 10% of the total optical energy of the LEDs can be used. Therefore, in order to increase the quantity of light, the drive current of the LED must be increased. For this reason, the power consumption of the LED array chips 208-l to 208-n and the LED drive ICs 209-l to 209-n and 210-l to 210-n is increased. Furthermore, ICs having higher driving capacities must be used. The substrate 202 serving as a radiating fin must be large, resulting in a higher cost and a bulky system.
FIG. 7 shows a mounting example of the imaging optical system 401. A mounting L-shaped block 505 is fixed to the substrate 202. The bottom and side walls of the imaging optical system 401 are fixed to inner surfaces 505' and 505" of the L-shaped block 505 with screws.
However, as described above, the light emission efficiency of the LEDs must be increased by using a focusing light guide array having a large angle .theta.. However, such a focusing light guide array having distances li and lo of 3 mm and distance lc of about 9 mm has a very large depth of field and allows formation of a clear image only within a focus error within .+-.100 .mu.m.
A focusing light guide array consists of a bundle of focusing rod lenses. Therefore, when an image of a small array such as LED light emitting surfaces is to be formed, if the array of LEDs is deviated from the central array of the bundle of lenses in a direction parallel thereto, the imaging surface has an undesirable density pattern at a pitch corresponding to the diameter of the focusing rod lenses.
As can be seen from FIG. 7, when thin films or the like are inserted between the inner surfaces 505' and 505" of the L-shaped block 505 and the focusing light guide array, the light emitting surfaces and the centers of the focusing light guides can be correctly adjusted at both sides of the array. However, the focusing light guide array has a dimension of 9 mm in the direction parallel to the optical axis thereof and a dimension of 4 mm in the vertical direction. The focusing light guide array must have a length of at least 300 mm in order to allow printing to a size of up to A3 size. Therefore, the portion near the center of the focusing light guide array is distorted. At times, it is difficult to keep the deviation between the center of the L-shaped block and that of the focusing light guide array within .+-.100 .mu.m. If a focusing light guide array is to be mounted with a good precision, a considerable cost increase cannot be avoided.
The light emitting surface of the conventional LED printer has the same width in the main scanning direction and in the subscanning direction as shown in FIG. 6. In particular, the light emitting surface width in the subscanning direction is set to be substantially the same as the dot pitch since the adjacent light emitting surface need not be formed.
In a printer of this type, a drum is rotated at a predetermined speed such that the array of light emitting surfaces and a photosensitive body, i.e., the surface of a photosensitive drum move relative to each other at a predetermined speed. Each time the light emitting surface array and the photosensitive body move for a distance corresponding to the dot pitch, a light emitting pattern of an array is switched to form a dot image. When a dot image of each array is formed, a correct black-and-white image can be formed with binary values of 0 and a predetermined value (corresponding to irradiated and non-irradiated portions of the photosensitive drum surface) if the light pattern is instantaneously emitted and if the light of the same shape as the light emitting surface is irradiated onto the photosensitive drum and the light emitting surface emits uniform light.
However, it is difficult to adopt a system wherein a light pattern is instantaneously emitted. The reason for this is as follows. When a single photosensitive drum is used, the required quantity of light is determined to be a product of illuminance and irradiation time. Therefore, when the light emitting time is reduced to 1/n, the illuminance must be increased to n times. This means that, in an LED array, a current which is n times the original current must be flowed, and a high current switching must be performed. In view of this, in general, a method is adopted wherein each light emitting element is turned on exceeding a predetermined period of time. In particular, in an LED array wherein driver chips are mounted on a head as shown in FIG. 2, the light emitting surface to be turned on is kept ON and the light emitting surface to be turned off is kept OFF while the light emitting surface array and the photosensitive drum move relative to each other for a distance corresponding to the dot pitch.
In this case, the light emitting surface width in the subscanning direction is substantially the same as the dot pitch. For this reason, the size and density of dots in the subscanning direction largely vary in accordance with variations in the illuminance of each light emitting surface.
These variations in the size and density of dots present a significant problem, particularly in a printer wherein the dot diameter during printing in the subscanning direction is changed in accordance with the length of ON time of each light emitting surface so as to provide a pseudo halftone image.